1. Field of the Invention
The present invention generally relates to a novel lighting circuit for a vehicular discharge lamp. More particularly, this invention pertains to a novel lighting circuit for a vehicular discharge lamp, that executes timing control in such a way as to generate a start pulse to the discharge lamp which has a correlation with the polarity of a rectangular wave pulse supplied to the discharge lamp and that can suppress the probability of deviation in the generation timing for the start pulse.
2. Description of the Related Art
To turn on a high-voltage discharge lamp, such as a metal halide lamp, it is necessary to generate a start pulse and supply it to the discharge lamp.
FIG. 14 shows an example of the structure of a conventional lighting circuit.
The lighting circuit a has a bridge circuit c which converts a DC voltage from a DC power supply circuit b to a rectangular wave voltage, and a start pulse generator d which generates a start pulse. The start pulse from the start pulse generator d is superimposed on the rectangular wave output from the bridge circuit c, and the resultant pulse is applied to a discharge lamp to activate the lamp.
The start pulse generator d has a power supply e, a transformer f, a switch element g, and a capacitor h, as shown in FIG. 15. When the terminal voltage of the capacitor h reaches a predetermined level, the switch element g is set on and the pulse generated then is boosted by the transformer f. The boosted pulse is superimposed on the output (rectangular wave) of the bridge circuit c and the resultant pulse is then applied to a discharge lamp i.
The bridge circuit c, though its detailed illustration is omitted, is so designed as to alternately switch two pairs of semiconductor switch elements to yield an AC output.
It is known that the easiness of the transition from the glow discharge of the discharge lamp i to the arc discharge varies depending on the phase relationship between the voltage direction of the start pulse and the polarity of the rectangular wave output from the bridge circuit c. Suppose that "V(1)" denotes the output voltage associated with one (j) of two power supply lines j and j', connecting the output terminals of the bridge circuit c to the power receiving terminals of the discharge lamp i, where the secondary winding of the transformer f is provided, and "V(2)" denotes the output voltage associated with the other power supply line j' as shown in FIG. 15. Then the lighting characteristic of the discharge lamp becomes better if the start pulse is generated in the direction indicated by an arrow A in FIG. 15 when the output voltage V(1) has a low level and the output voltage V(2) has a high level.
There are two possible ways to generate the start pulse at such a timing. The first method is to provide a switch element having a trigger terminal and its control circuit and to execute synchronous control in such a way that the switch element g is set on only when V(2) is at a high level. The second method is to use a self-breakdown switch element, such as a spark gap, for the switch element g so that the capacitor is charged only in a specific phase of the rectangular wave.
The former method however requires a high-breakdown switch element and its driving circuit and/or control circuit, thus complicating the circuit structure. In this respect, the latter method is practically used and may employ a circuit structure as shown in FIG. 16.
A start pulse generator k has a constant power supply circuit l, a transformer m, a self-breakdown switch element n and a capacitor o.
The primary winding and the secondary winding of the transformer m are wound in the opposite phases, with the secondary winding connected to one (j) of the power supply lines j and j' which connect the output terminals of the bridge circuit c to the power receiving terminals of the discharge lamp i. The primary winding of the transformer m has a winding-start end connected to one end of the self-breakdown switch element n and also connected to the winding-termination end of the secondary winding of the transformer m. The winding-termination end of the primary winding is connected via the capacitor o to the other end of the self-breakdown switch element n.
The constant power supply 1 has a positive terminal connected between the self-breakdown switch element n and the capacitor o via a resistor p and a forward biased diode q, and the other terminal connected to the power supply line j'.
Given that "v" denotes the amplitude of the rectangular wave from the bridge circuit c and "el" denotes the voltage from the constant power supply I, the charge voltage to the capacitor o becomes "el-v" when the voltage v(1) associated with the power supply line j is at a high level and becomes "el+v" when the voltage v(2) associated with the power supply line j' is at a high level. That is, the charge voltage varies by the phase of the rectangular wave.
When the self-breakdown switch element n is designed to yield with the voltage el, the terminal voltage Vc of the capacitor o rises only in the high-level duration of V(2) as shown in FIG. 17 and the self-breakdown switch element n yields only in that period. The pulse generated at this time is boosted by the transformer m and the boosted pulse is superimposed on the rectangular wave output frown the bridge circuit c. The resultant pulse is then applied to the discharge lamp i.
The self-breakdown switch element does not yield immediately when the terminal voltage of the capacitor reaches a predetermined level, but functions with a certain delay time. This affects the relationship between the timing of generating the start pulse and the phase of the rectangular wave, so that the start pulse generator may not be generated at the given timing.
While it is ideal that the self-breakdown switch element n should yield at a time ta when the terminal voltage of the capacitor o reaches el as shown in FIG. 17, the switch element n may actually yield at a time tb with a delay t from the time ta and the time tb may be shifted into the next half cycle (where V(1) is at a high level). In this case, it may not be possible to generate the start pulse when V(2) has a high level, disadvantageously.